1. Field of the Invention
This invention relates in general to a method and apparatus for improving the picture quality in differential pulse code modulation transmitting systems and in particular to method and apparatus for producing reconstructed samples which utilize auxiliary samples associated with the blanking interval at both the receiver and the transmitter.
2. Description of the Prior Art
Differential pulse code modulation (DPCM) is utilized in the transmission of video signals for reducing the data rate. Such DPCM systems are disclosed in the book entitled "Digital Television" by R. H. Stafford, published by John Wiley & Sons (1980), at pages 30 through 58. The DPCM coder calculates an estimated value of data from the neighboring samples of a current sample of data and compares this estimated value to the present sample and outputs the quantitized difference between these two values as DPCM signal value. The receiver reconstructs the samples using the received quantized DPCM signal values. So as to keep the quantitized error from resulting in too large an error between the reconstructed samples and the original samples, the DPCM coder at the transmitter using a DPCM method is constructed such that a recursive DPCM decoder is contained therein, in other words, the same estimated values are used at the transmitter as well as the receiver. The recursive apparatus also assures that the mean value of the quantitized value is zero.
In multi-dimensional DPCM, X samples adjacent the current sample are utilized for calculating the estimated value s. In two dimensional DPCM the sample A which preceded the current sample, the sample B of the preceding picture line which lies directly above the sample A and samples C and D following are frequently employed as the neighboring samples. In three dimensional DPCM coding, the neighboring samples of the preceding picture or field are also utilized.
The codings of the last DPCM value of a picture line and the reconstruction at the receiver of the last sample of a picture line are problemmatical because only the DPCM values of the scanning values (picture points) of the active lines are transmitted to the receiver. The DPCM values .DELTA.s for scanning values already falling into the blanking gap are not available to the receiver and must be replaced by an optional value. Thus, the effect of this error in reconstructive scanning values also extends into the succeeding active picture lines since the scanning value is also used in the reconstruction of scanning values of the next picture line and so forth according to the picture content. This error results from the use of different estimated values at the transmitting and receiving locations.